The isolated perfused kidney of the rat will be used to study the nature of the pumps responsible for active reabsorption of glomerular filtrate, the relationship between energy utilization and work performed by the kidney, the mechanism of the effect of Starling forces on tubular reabsorption, the relationship between glucose metabolism and sodium transport, and the possible connection between these factors and the accumlation of edema in certain disease states. The role of Na-K-ATPase in kidney function will be investigated by applying specific ultramicrochemical techniques to individual cells or segments of tubules microdissected from frozen, lyophilized sections of rat kidney. This method, not previously applied in this country to the study of Na-K-ATPase, has the general virtue of eliminating problems related to the cellular heterogeneity of the kidney. The locus and mechanism of hormone action, especially that of aldosterone, will be the focus of initial studies. Comparative physiological studies of the teleost gill elasmobranch rectal gland will investigate these biological models of active chloride transport.